Fusee (horology)

The primitive verge and foliot timekeeping mechanism, used in all early clocks, was sensitive to changes in drive force.

So early spring-driven clocks slowed down over their running period as the mainspring unwound, causing inaccurate timekeeping.

As the movement ran, the tapering shape of the fusee pulley continuously changed the mechanical advantage of the pull from the mainspring, compensating for the diminishing spring force.

Fusees became the standard method of getting constant force from a mainspring, used in most spring-wound clocks, and watches when they appeared in the 17th century.

Around 1726 John Harrison added the maintaining power spring to the fusee to keep marine chronometers running during winding, and this was generally adopted.

When it reaches the top, it presses against a lever, which moves a metal blade into the path of a projection sticking out from the edge of the fusee.

[13] The fusee was a good mainspring compensator, but it was also expensive, difficult to adjust, and had other disadvantages: Achieving isochrony was recognised as a serious problem throughout the 500-year history of spring-driven clocks.

The invention of the pendulum and the balance spring in the mid-17th century made clocks and watches much more isochronous, by making the timekeeping element a harmonic oscillator, with a natural "beat" resistant to change.

In 1760, Jean-Antoine Lépine dispensed with the fusee, inventing a going barrel to power the watch gear train directly.

By 1850, the Swiss and American watchmaking industries employed the going barrel exclusively, aided by new methods of adjusting the balance spring so that it was isochronous.

After this, the only remaining use for the fusee was in marine chronometers, where the highest precision was needed, and bulk was less of a disadvantage, until they became obsolete in the 1970s.